The present invention relates to hydrometallurgy. More particularly, it relates to the recovery of metals from aqueous solutions by extraction techniques and subsequent scrubbing and stripping of the metal-loaded extractant.
The extraction of cobalt(II) from leach liquors containing nickel(II), by means of water-immiscible organic solutions containing bis(2-ethylhexyl)hydrogen phosphate, is well-known in the art. However, the relatively poor selectivity of this reagent has limited its application by solvent extraction to leach liquors containing nickel(II)/cobalt(II) ratios below 5. Even in these liquors, the process requires expensive, multistage contacting devices to achieve a cobalt (II)/nickel(II) ratio of about 1,000 in the final product.
With leach liquors containing higher nickel(II)/cobalt(II) ratios, for example 50/l, separation of cobalt(II) by solvent extraction can only be achieved using amines and a chloride leach liquor. However, in the chloride systems corrosion is a problem. Alternatively, the Outokumpu Oy hydrometallurgical process may be used, but it is expensive.
The major advantage of the process of Belgian Pat. No. 871,963 over the use of bis(2-ethylhexyl)hydrogen phosphate lies in the much greater selectivity of cobalt(II) extraction over nickel(II) extraction by the use of phosphonates. The separation factor obtained with the phosphonates is an order of magnitude higher than that obtained with bis(2-ethylhexyl)hydrogen phosphate. There is still a need, however, for other extractants that have high selectivity for the separation of cobalt(II) from nickel(II), and will allow most of the cobalt(II) to be stripped from the extract with a mineral acid.